Many of today's microcontroller integrated circuits have a "power on reset (POR)" feature. The purpose of this power on reset feature is to ensure that the circuits in a microcontroller are initialized to a known state each time a positive power supply voltage is applied to the power integrated circuit pin or pins of the microcontroller. The term "positive power supply voltage" as used in this document refers to a power supply voltage which is positive with respect to ground.
The power on reset feature can also be used to control the initialization of a crystal oscillator. In most applications utilizing a microcontroller, a crystal oscillator and its associated logic are commonly used to generate clocks signals. These clock signals are then used to clock various circuits in a microcontroller. One way the power on reset feature achieves the desired initialization is to ensure that the crystal oscillator receives an adequate amount of time to start and stabilize before the generated clocks signals are used to clock various circuits in the microcontroller. Thus the power on reset feature can be used to ensure that the clock signals are sufficiently stable before they are used.
A "power on detection" circuit is utilized in some microcontrollers as part of the implementation of the power on reset feature. The power on detection circuit is a circuit which is used to detect a "power on condition". A "power on condition" occurs when a positive power supply voltage is applied to one or more power integrated circuit pins of the microcontroller.
Some prior art power on detection circuits have serious drawbacks. For example, some circuits are unable to detect a power on condition when the rise time of the positive power supply voltage is very slow. Also, some power on detection circuits draw too much DC current. In addition, some power on detection circuits require the use of a large resistor, which may require a significant amount of semiconductor area.